DESCRIPTION (Applicant's abstract): Stable isotope medical diagnostic breath tests enjoy the advantages of being non-invasive, highly specific to detecting an invading organism or measuring organ function, and usually lower cost than competing tests. However, having a lower cost is not always sufficient. There is great financial pressure and incentives to introduce tests that dramatically reduce health care expenditures. Reverse Isotope Dilution (RID) was developed to significantly reduce the cost of future breath tests and in the process increase the array of possible breath test applications. The novelty of this application is the RID concept of administering a low cost tracer with an unlabelled substrate probe. The unlabeled substrate probe dilutes the CO2 excretion curve that would be expected had only the labeled tracer been administered. The degree to which the curve is modified provides insight into the functionality of a specific enzymatic pathway. With RID many different tests can be performed using the same labeled tracer but only changing the unlabeled probe. The benefits include making breath tests economically attractive and expanding the possible applications for tracer tests. Our long-term objective is to use RID technology to develop a family of gastrointestinal stable isotope breath tests that are clinically useful and low cost.